Machines eyeball trash for single-stream recycling

CNET's Martin LaMonica follows recycling waste on the next step of its life: a high-tech, single-stream recycling center where machines do the bulk of the heavy lifting.

Martin LaMonica Former Staff writer, CNET News
Martin LaMonica is a senior writer covering green tech and cutting-edge technologies. He joined CNET in 2002 to cover enterprise IT and Web development and was previously executive editor of IT publication InfoWorld.
Martin LaMonica
5 min read

BOSTON--Who knew a warehouse full of garbage and conveyor belts could be so high-tech?

Last Thursday, I had the chance to tour a single-stream recycling center here, where a suite of specialized machines has vastly sped up the traditional manual sorting method now commonly used in recycling.

Well over 100 full-time employees still work at the plant, but the bulk of the separation work is done by a combination of belts, magnets, and optical readers clever enough to tell the difference between a plastic milk jug and a laundry detergent container.

The tour opened my eyes to how single stream recycling, where all recyclables are placed into a single container, works once your castaways leave the curb side. Automation can play a big role, making life a lot simpler for consumers and boosting recycling rates significantly.

Digging through a high-tech recycling center (photos)

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Vermont-based Casella Waste Systems retrofitted an existing recycling facility in an industrial zone in the Charlestown neighborhood of Boston in 2009, converting it from a dual-stream facility--one that takes paper separated from everything else--into single-stream.

Last year, the company added more capacity to handle the growing volume, as it gains more customers. It's now handling about 16,000 tons a month, according to Lisa McMenemy who led the tour, which was organized by a nonprofit group of chemical engineers in the Boston area. To visualize a ton of trash, consider that a large garbage can or wheeled barrel can hold roughly 200 pounds, according to the Charlestown plant manager. So a ton is about 10 large, full cans.

For many Americans, recycling is already a routine practice, but higher recycling rates pay off in a number of ways. For starters, municipalities pay less to garbage haulers by reducing the amount of tonnage that goes to landfills or incinerators. The City of Boston estimated that it saved about $1 million a year when it started in 2009.

Consumers, too, don't need to pay as much attention to what goes where, making it easier to recycle more material. Recycling a product for a second (or third or fourth) life means less energy is used to extract and process virgin materials and helps save natural resources such as timber, water, and minerals, according to the EPA.

When it comes to household waste, 60 percent is recyclable, according to McMenemy. Another 30 percent is organic waste, such as food and yard waste, which could be repurposed through composting or, as San Francisco does, collected and turned into fertilizer.

Optical eyes, magnets, and air guns
Only 10 or 15 years ago, the Casella Charlestown facility was just one large conveyor belt with lots of people picking out recyclables. Its investment in an automated single-sort system means that 75 percent of the material is now sorted by machine.

As the trash passes through different machines, different types of refuse, such as newspaper and cardboard, are separated from the main stream and put into separate bins. Once collected, unified waste streams, such as aluminum cans, are compacted into large bales and then sold through regular contracts or on-the-spot market for recycled material.

Crushed aluminum cans, automatically separated from mixed recycling, await the compressor where they will be packaged as giant bales and sold to mills.
Crushed aluminum cans, automatically separated from mixed recycling, await the compressor where they will be packaged as giant bales and sold to mills. Martin LaMonica/CNET

The front end of the process is essentially a mountain of garbage, dumped by recycling trucks as they enter the warehouse. From there, it's dropped--unsorted--onto the first of many conveyor belts by a small tractor, or a skid steer.

The first stop is where most of the employees work--pre-sorting. People put all sorts of things that should not go into recycling, such as pots, pans, dog chains, and other metal items. The worst offender here, though, is plastic shopping bags, which get caught in the moving parts and regularly bring the operation to a halt. (Plastic bags, if you use them, can often be recycled at supermarkets.)

Workers simply grab junk that doesn't belong there and put it aside where it will be either sent to a landfill or collected for scrap metal. If the bales that Casella sells to mills have too many contaminants, they can be rejected.

This stream of waste passes over a series of spinning discs. Since cardboard can't fit through the cracks in the discs, which resemble a big screw, the cardboard sheets break off onto their own conveyor belt and are dropped onto the floor below for collection. Another set of screens allows the heavier plastic and metal materials to fall to another belt while much of the paper continues on to a separate bin.

Most of the glass is quickly separated, too, as garbage passes through a large spinning tube, which sends the glass crashing onto the perforated floor of the tube and into collectors below. Tin gets snatched by large magnets en route. Later in the process, another type of magnetic field called an eddy current yanks aluminum cans from a fast-flowing conveyor belt. Instead of dropping straight down, aluminum materials (including foils) jump into another collection bin.

With mostly just plastics left, machines use optical readers to identify one type from another. Machine programmers spent days "teaching" the infrared optical readers to recognize different plastic types based on more than 100 different parameters, including the color, density, and shape. Once the machine knows that a number 3 plastic is coming, a few feet later it shoots a jet of air to blow it off the conveyor belt and into a collection bin.

Standing there watching it for a few minutes, you can hear a steady stream of these rapid bursts of air picking off different items and sending them flying off in another direction. The plant manager said it took technicians five days to optimize the computer recognition, which is still not perfect since 25 percent of the material is sorted by hand.

At that point, most everything is separated and a large vibrating machine called a destoner blows the remaining bits of paper off from the remaining stream and collects the "residue," such as small shards of glass and other material that can't be recycled.

At the back of the warehouse is a compressor to create bales of all the different materials, including the residue, which are stacked over 20 feet high. There they wait for the next stage of their life.